I hope this helps you
а)
1.53х+0.47x=15
2x=15
x=7.5
б)
3.28х+4.72х=17
8х=17
х=17/8=2.125
<span>19*21= (20-1) (20+1) =20^2 -1 = 400- 1 =399</span>